More than one hundred and fifty years ago when the earth passed through the long tail of the Comet Tempel-Tuttle, many observing the meteor storm that followed thought the world was ending.
Today, the scientific community at the Air Force Research Laboratory’s
Starfire Optical Range, here, greeted that same meteor storm with eager
anticipation.
Those famous “shooting stars,” which in the past, rained down on the earth by the thousands, were expected to assist in yielding valuable information about this planet’s upper atmosphere.
The purpose of the experiments, using a laser radar system (LIDAR), was to measure sodium about 60 miles above the earth in the upper atmosphere. It is hoped that the data will provide more information about the chemistry and physics at that level of the atmosphere. Results of this information, in turn, might allow future adaptive optics to produce higher resolution images of space objects and satellites.
In the past, scientists, wanting to know more about the upper atmosphere,
have had to depend on expensive methods of data collection including rockets to the upper atmosphere.
In this case, nature has provided the Leonid meteors that are known to
create rare contrails. It was those contrails the experimenters targeted in mid-November.
Dr. Jack Drummond, a Directed Energy Directorate astronomer, said in the
past, meteor trails have only been caught accidentally and the whole sum of past data equates to about 10 or 12 minutes of material.
“This is the first time anyone has every tried to chase the meteors. We were able to capture more than an hour of data from ten meteors over two nights,” he said.
The crystal clear sky created a perfect backdrop for the light show that
peaked during the early morning hours before dawn. Above the “oohs” and
“aahs” of the group, as the meteors streaked across the sky, was the voice of the spotter, Dr. Drummond, providing azimuth and altitude to the telescope operator.
Not every meteor created a contrail. When a contrail did appear, the spotter guided the 3.5-meter telescope across the darkened sky until the laser picked it up.
Drummond said the longest previous measurement of a meteor trail was only
about a minute or two. “We had two contrails that lasted more than a half
hour each. It was spectacular,” he said enthusiastically.
During the experiment, the LIDAR was used to excite the neutral sodium
atoms.
“We shot the LIDAR at a certain frequency and it pumped up an electron one level. When that one electron dropped back one level, it emitted that same frequency of light and that is what we were seeing,” Drummond explained.
What they were able to see will help the scientists understand wind
velocity, wind shear, diffusion, temperature and density in the upper
atmosphere.
Drummond said one thing they noted during the experiment was there doesn’t seem to be a uniform wind velocity field in the upper atmosphere. “One meteor contrail moved in a southeast direction,” he said. “We expected the next one to do the same. Instead, the next contrail moved north.”
Another area of interest was an inexplicable enhancement of the amount of
sodium deposited in comparison to the size of the particle. “Something is
happening in physics and chemistry at that altitude that we don’t fully
understand,” he added.
Drummond said the laboratory is involved in a cooperative agreement with
two universities. The University of Illinois and Cornell University are
providing equipment and expertise for special observation projects over a
three-year period. Together the group decided to study the Leonid meteor
contrails using a LIDAR, on loan from the University of Illinois.
According to Drummond, “We increased the amount of data available by an
order of magnitude — by a factor of ten.” The directorate scientist
speculated that enough data was collected in one night to provided
dissertation material for three graduate students.
Drummond also said he is not aware that anyone else has ever video taped an evolving meteor trail. He explained that there were two different types of trails recorded.
One type was evanescent, expanded quickly, and was optically thin. “You
could see right through it,” he explained.
The other example was optically thick, did not evolve very fast, and was
much brighter. There was also no detection of sodium enhancement in this
trail.
Drummond said PhD candidates from the two universities would be analyzing
the data. He said 1999 might be an even better year for gathering data on
the Leonid meteors.
Leonids on the Internet
Leonids at Spacer.Com
